Selective null transmission circuit



Patented Mar. 27, 1945 SELECTIVE NULL TRANSMISSION CIRCUIT John R. Ford, Narberth, and John M. Bruinbaugh, Lansdowne, Pa., minors to Radio Corporation of America, a corporation of Delaware Application April 30, 1942, Serial No. 441,206

11 Claims.

This invention relates generally to tuned transmission circuits and particularly to selective attenuation circuits providing substantially flat response to frequencies throughout an extended band except at one selected frequency at which th circuit exhibits a sharp attenuation characteristic.

Similar prior art circuits have had the d sadvantage that the attenuation increased rather gradually as the input frequency approached a predetermined frequency. This invention to a great extent overcomes this difilculty.

The invention is designed to take advantage of the combined characteristics of a degenerative feedback circuit in which no degeneration, and consequently maximum gain, is present at the desired frequency. and the normal attenuation characteristic of the filter circuit utilized for tuning the degenerative circuit. This arrangement provides considerably greater selectivity at the desired frequency than can be normally obtained with a selected filter circuit since the increasin gain of the degenerative amplifier tends to hold up the overall response as the circuit approaches the frequency of maximum filter attenuation.

Among the objects of the invention are to provide a simple transmission circuit havin improved means for providing considerable attenuaton at a selected frequency within the transmission band of the apparatus. Another object is to provide an improved transmission circuit including an amplifier having degenerative feedback which is a function of the attenuation characteristics of a selected filter circuit, and means for deriving from the output of the filter circuit a s gnal, the attenuation of which is proportional to the combined frequency characteristics of the amplifier and the filter circuit. Another object of the invention is to provde an improved transmission circuit including an amplifier having degenerative feedback which is a function of the attenuation characteristics of a selected adjustable filter circuit, and means for deriving from the output of the adjustable filter circuit a signal. the attenuation of which is proportional to the combined frequency characteristics of the amplifier and the adjustable filter circuit. Another object is to provide an improved transmission c=rcuit includ ng an amplifier having degenerative feedback which is a function of the attenuation characteristics of a circuit in which the frequency at which maximum attenuation exists may be varied, comprising a resistancescapacit-y net ork, and means for deriving from the output o the attenuation circuit a signal, the attenuation of which i proportional to the combined frequency characteristics of the amplifier and the resistance-capacity network.

The invention will be described by reference to the drawing, of which Figure 1 is a block diagram of one embodiment of the invention, Figure 2 is a schematic circuit diagram of a preferred embodiment of the invention and Figure 3 is a graph illustrating the operating characteristics of the invention. Similar reference numerals are applied to similar elements throughout the drawing.

The circuit of Fig. 1 includes an amplifier 2 having an input signal applied to it inputterminals I. The output of the amplifier is applied to a filter circuit having maximum attenuation at the desired frequency, and capable of transmittin all other frequencies within the desired transmission band. The output of the filter 3 connected to the circuit output terminals 5 and through a suitable degenerative feedback circuit 4 to the input circuit of the amplifier 2.

The attenuation of the circuit is the product of the transmission characteristics of the degenerative amplifier 2 including the filter 3, and the attenuation provided by the filter circuit 3 to the signal derived from the amplifier 2. The circuit provides a sharp attenuation at the desired frecathode load which, in a preferred embodiment,

is a second amplifier tube 9, which may be of the RCA 1852 Or equivalent type. The second amplifier tube 9 includes in its anode circuit a re sistor-capacitor network 3 including the series resistors RI and R2 connected in parallel with the series capacitors Cl, C2. A capacitor C3 is connected between the junction of the resistors RI and R2 and ground. A resistor R3 is connected between the junction of the capacitors Cl and C2 and ground. Thi type of circuit provides a maximum attenuation at a selected frequency determined by the relative values of the circuit components. This frequency may be readily varied by providing variable resistors at Ri, R2 and R3. Maximum attenuation occurs when if CI=C2; C3=4Cl; and RI=R2=R3. The feedback ratio of the circuit is provided by a voltage divider comprising series resistors i3 and i4 connected between the output or the filter circuit l3 and ground. The Junction of the resistors l3 and i4- is connected to the control grid of the second amplifier tube 9, to provide degenerative feedback. Normally, the anode resistance of the tube 9 should be high with respect to the value of a resistor ll connected between the cathode of the first amplifier tube I and the anode of the second amplifier tube 9. It will be apparent that the degenerative feedback voltage, introduced by the anode of the amplifier tube 9, will oppose the normal signal voltage impressed on the anode circuit of this tube.

Fig. 3 includes a first graph F representing the attenuation characteristic oi the filter circuit 3. A second graph N represents the attenuation characteristic of the degenerative feedback amplifier including the second amplifier tube 9 and the filter circuit 3, while a third graph C indicates the product of the combined attenuation of the degenerative amplifier and the filter circuit provided by the invention.

We claim as our invention:

1. A signal transmission circuit including an attenuation circuit providing maximum attenuation at a predetermined operating frequency and an amplifier having input and output circuits. means for applying a signal to said input circuit, means for applying signals derived from said output circuit to said attenuation circuit, aperiodic degenerative means for applying signals derived from said attenuation circuit to said amplifier input circuit, and means for utilizing the signals transmitted by said attenuation circuit.

2. A signal transmission circuit including an attenuation circuit providing maximum attenuation at a predetermined operating frequency and an amplifier having input and output circuits, means for applying a signal to said input circuit, means for applying signals derived from said output circuit to said attenuation circuit, aperiodic degenerative means for coupling signals derived from said attenuation circuit to said amplifier, and means for deriving S als of amplitude dependent on the combined transmission frequency characteristics of said amplifier and said attenuation circuit.

3. A signal transmission circuit including an attenuation circuit comprising a resistor-capacitor network providing maximum attenuation at a predetermined operating frequency and an emplifier having input and output circuits, means for applying a signal to said input circuit, means erative means for coupling signals derived from said attenuation circuit to said amplifier, and

'means for deriving signals of amplitude dependtenuation at a predetermined operating frequency and an amplifier having input and output circuits, means for applying a signal to said input circuit, means for applying signals derived from said output circuit to said attenuation circuit,

aperiodic degenerative means for coupling signals derived from said attenuation circuit to said amplifier input circuit and means for utilizing the signals transmitted by said attenuation circuit.

6. A signal transmission circuit including an attenuation circuit comprising an adjustable resistor-capacitor network providing maximum attenuation at a predetermined operating frequency and an amplifier having input and output circuits, means for applying a signal to said input circuit, means for applying signals derived from said output circuit to said attenuation circuit, aperiodic degenerative means for coupling signals derived from said attenuation circuit to said amplifier, and means for deriving signals of amplitude dependent on the combined transmission 'frequency characteristics of said amplifier and said attenuation circuit.

7. A signal transmission circuit including an attenuation circuit comprising an adjustable resister-capacitor network providing maximum attenuation at a. predetermined operating frequency for applying signals derived from said output for applying signals derived from said output circuit to said attenuation circuit, aperiodic degenand an amplifier having input and output circuits, means for applying a signal to said input circuit, means for applying signals derived from said output circuit to said attenuation circuit, aperiodic degenerative means variable with frequency for coupling said signals to said amplifier input circuit and means for utilizing the signals transmitted by said attenuation circuit.

8. A signal transmission circuit including an attenuation circuit comprising an adjustable resistor-capacitor network providing maximum attenuation at a predetermined operating frequency and an amplifier having input and output circuits, means for'applying a signal to said input circuit, means for applying signals derived from said output circuit to. said attenuation circuit, aperiodic degenerative means variable with frequency for coupling said signals to said amplifier,

and means for deriving signals of amplitude dependent on the combined transmission frequency characteristics of said amplifier and said attenuation circuit.

9. A signal transmission circuit including an attenuation circuit providing maximum attenuation at a predetermined operating frequency and an amplifienmeans for applying a signal to said amplifier, means for applying said amplified signals to said attenuation circuit, aperiodic degenerative means in said amplifier responsive to signals derived from said attenuation circuit for controlling the selectivity of said amplifier, and means for deriving signals combining the selectivity characteristics of said amplifier andsaid attenuation circuit.

10. A signal transmission circuit including an attenuation circuit comprising a resistor-capacitor network providing maximum attenuation. at.

a predetermined operating frequency and an amplifier, means for applying a signal to said amplifier, means ior applying said amplified signals to said attenuation circuit, aperiodic degenerative means in said amplifier responsive to signals derived from said attenuation circuit for controlling the selectivity of said amplifier and means for deriving dgnala combining the selectivity characteristics of said amplifier and said attenuation circuit.

11. A signal transmission circuit including an attenuation circuit comprising an adjustable resister-capacitor network providing maximum attenuation at a predetermined operating frequency and an amplifier, means for applying a signal to said amplifier, means for applying said amplified signals to said attenuation circuit, aperiodic degenerative means in said amplifier responsive to signals derived from said attenuation circuit for controlling the selectivity 0! said amplitier, and means i'or deriving signals combining the selectivity characteristics of said amplifier and said attenuation circuit.

JOHN R. FORD. JOHN H. BRUHBAUGH. 

